Conventionally, as disclosed in JP-A-2001-225666, an eye state detection apparatus, which detects an opening degree of an eye of a driver according to a face image taken by a camera, is known. The eye state detection apparatus detects the opening degree of the eye based on an opening degree reference value, which is obtained by a learning.
As described above, in JP-A-2001-225666, the eye state detection apparatus detects the opening degree of the eye based on the opening degree reference value. Thus, in a case where a state of the eye is in a predetermined state, the opening degree reference value is set improperly so that the eye state detection apparatus fails to accurately detect an open state of the eye. The predetermined state of the eye may include a state in which the eye is in a closed state for a relatively long time.